/*
 * Copyright (c) 2006 and onwards Makoto Yui
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package btree4j.utils.io;

import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

/**
 * A <code>BufferedInputStream</code> adds functionality to another input stream-namely, the ability
 * to buffer the input and to support the <code>mark</code> and <code>reset</code> methods. When the
 * <code>BufferedInputStream</code> is created, an internal buffer array is created. As bytes from
 * the stream are read or skipped, the internal buffer is refilled as necessary from the contained
 * input stream, many bytes at a time. The <code>mark</code> operation remembers a point in the
 * input stream and the <code>reset</code> operation causes all the bytes read since the most recent
 * <code>mark</code> operation to be reread before new bytes are taken from the contained input
 * stream.
 *
 * @version 1.57, 06/07/06
 */
public final class FastBufferedInputStream extends FilterInputStream {

    private static int defaultBufferSize = 8192;

    /**
     * The internal buffer array where the data is stored. When necessary, it may be replaced by
     * another array of a different size.
     */
    protected volatile byte buf[];

    /**
     * Atomic updater to provide compareAndSet for buf. This is necessary because closes can be
     * asynchronous. We use nullness of buf[] as primary indicator that this stream is closed. (The
     * "in" field is also nulled out on close.)
     */
    private static final AtomicReferenceFieldUpdater<FastBufferedInputStream, byte[]> bufUpdater =
            AtomicReferenceFieldUpdater.newUpdater(FastBufferedInputStream.class, byte[].class,
                "buf");

    /**
     * The index one greater than the index of the last valid byte in the buffer. This value is
     * always in the range <code>0</code> through <code>buf.length</code>; elements
     * <code>buf[0]</code> through <code>buf[count-1]
     * </code>contain buffered input data obtained from the underlying input stream.
     */
    protected int count;

    /**
     * The current position in the buffer. This is the index of the next character to be read from
     * the <code>buf</code> array.
     * <p>
     * This value is always in the range <code>0</code> through <code>count</code>. If it is less
     * than <code>count</code>, then <code>buf[pos]</code> is the next byte to be supplied as input;
     * if it is equal to <code>count</code>, then the next <code>read</code> or <code>skip</code>
     * operation will require more bytes to be read from the contained input stream.
     *
     * @see java.io.BufferedInputStream#buf
     */
    protected int pos;

    /**
     * The value of the <code>pos</code> field at the time the last <code>mark</code> method was
     * called.
     * <p>
     * This value is always in the range <code>-1</code> through <code>pos</code>. If there is no
     * marked position in the input stream, this field is <code>-1</code>. If there is a marked
     * position in the input stream, then <code>buf[markpos]</code> is the first byte to be supplied
     * as input after a <code>reset</code> operation. If <code>markpos</code> is not <code>-1</code>
     * , then all bytes from positions <code>buf[markpos]</code> through <code>buf[pos-1]</code>
     * must remain in the buffer array (though they may be moved to another place in the buffer
     * array, with suitable adjustments to the values of <code>count</code>, <code>pos</code>, and
     * <code>markpos</code>); they may not be discarded unless and until the difference between
     * <code>pos</code> and <code>markpos</code> exceeds <code>marklimit</code>.
     *
     * @see java.io.BufferedInputStream#mark(int)
     * @see java.io.BufferedInputStream#pos
     */
    protected int markpos = -1;

    /**
     * The maximum read ahead allowed after a call to the <code>mark</code> method before subsequent
     * calls to the <code>reset</code> method fail. Whenever the difference between <code>pos</code>
     * and <code>markpos</code> exceeds <code>marklimit</code>, then the mark may be dropped by
     * setting <code>markpos</code> to <code>-1</code>.
     *
     * @see java.io.BufferedInputStream#mark(int)
     * @see java.io.BufferedInputStream#reset()
     */
    protected int marklimit;

    /**
     * Check to make sure that underlying input stream has not been nulled out due to close; if not
     * return it;
     */
    private InputStream getInIfOpen() throws IOException {
        InputStream input = in;
        if (input == null)
            throw new IOException("Stream closed");
        return input;
    }

    /**
     * Check to make sure that buffer has not been nulled out due to close; if not return it;
     */
    private byte[] getBufIfOpen() throws IOException {
        byte[] buffer = buf;
        if (buffer == null)
            throw new IOException("Stream closed");
        return buffer;
    }

    /**
     * Creates a <code>BufferedInputStream</code> and saves its argument, the input stream
     * <code>in</code>, for later use. An internal buffer array is created and stored in
     * <code>buf</code>.
     *
     * @param in the underlying input stream.
     */
    public FastBufferedInputStream(InputStream in) {
        this(in, defaultBufferSize);
    }

    /**
     * Creates a <code>BufferedInputStream</code> with the specified buffer size, and saves its
     * argument, the input stream <code>in</code>, for later use. An internal buffer array of length
     * <code>size</code> is created and stored in <code>buf</code>.
     *
     * @param in the underlying input stream.
     * @param size the buffer size.
     * @exception IllegalArgumentException if size is less than or equals to 0.
     */
    public FastBufferedInputStream(InputStream in, int size) {
        super(in);
        if (size <= 0) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }

    /**
     * Fills the buffer with more data, taking into account shuffling and other tricks for dealing
     * with marks. Assumes that it is being called by a synchronized method. This method also
     * assumes that all data has already been read in, hence pos > count.
     */
    private void fill() throws IOException {
        byte[] buffer = getBufIfOpen();
        if (markpos < 0)
            pos = 0; /* no mark: throw away the buffer */
        else if (pos >= buffer.length) /* no room left in buffer */
            if (markpos > 0) { /* can throw away early part of the buffer */
                int sz = pos - markpos;
                System.arraycopy(buffer, markpos, buffer, 0, sz);
                pos = sz;
                markpos = 0;
            } else if (buffer.length >= marklimit) {
                markpos = -1; /* buffer got too big, invalidate mark */
                pos = 0; /* drop buffer contents */
            } else { /* grow buffer */
                int nsz = pos * 2;
                if (nsz > marklimit)
                    nsz = marklimit;
                byte nbuf[] = new byte[nsz];
                System.arraycopy(buffer, 0, nbuf, 0, pos);
                if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
                    // Can't replace buf if there was an async close.
                    // Note: This would need to be changed if fill()
                    // is ever made accessible to multiple threads.
                    // But for now, the only way CAS can fail is via close.
                    // assert buf == null;
                    throw new IOException("Stream closed");
                }
                buffer = nbuf;
            }
        count = pos;
        int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
        if (n > 0)
            count = n + pos;
    }

    /**
     * See the general contract of the <code>read</code> method of <code>InputStream</code>.
     *
     * @return the next byte of data, or <code>-1</code> if the end of the stream is reached.
     * @exception IOException if this input stream has been closed by invoking its {@link #close()}
     *            method, or an I/O error occurs.
     * @see java.io.FilterInputStream#in
     */
    public int read() throws IOException {
        if (pos >= count) {
            fill();
            if (pos >= count)
                return -1;
        }
        return getBufIfOpen()[pos++] & 0xff;
    }

    /**
     * Read characters into a portion of an array, reading from the underlying stream at most once
     * if necessary.
     */
    private int read1(byte[] b, int off, int len) throws IOException {
        int avail = count - pos;
        if (avail <= 0) {
            /* If the requested length is at least as large as the buffer, and
               if there is no mark/reset activity, do not bother to copy the
               bytes into the local buffer.  In this way buffered streams will
               cascade harmlessly. */
            if (len >= getBufIfOpen().length && markpos < 0) {
                return getInIfOpen().read(b, off, len);
            }
            fill();
            avail = count - pos;
            if (avail <= 0)
                return -1;
        }
        int cnt = (avail < len) ? avail : len;
        System.arraycopy(getBufIfOpen(), pos, b, off, cnt);
        pos += cnt;
        return cnt;
    }

    /**
     * Reads bytes from this byte-input stream into the specified byte array, starting at the given
     * offset.
     *
     * <p>
     * This method implements the general contract of the corresponding
     * <code>{@link InputStream#read(byte[], int, int) read}</code> method of the
     * <code>{@link InputStream}</code> class. As an additional convenience, it attempts to read as
     * many bytes as possible by repeatedly invoking the <code>read</code> method of the underlying
     * stream. This iterated <code>read</code> continues until one of the following conditions
     * becomes true:
     * <ul>
     *
     * <li>The specified number of bytes have been read,
     *
     * <li>The <code>read</code> method of the underlying stream returns <code>-1</code>, indicating
     * end-of-file, or
     *
     * <li>The <code>available</code> method of the underlying stream returns zero, indicating that
     * further input requests would block.
     *
     * </ul>
     * If the first <code>read</code> on the underlying stream returns <code>-1</code> to indicate
     * end-of-file then this method returns <code>-1</code>. Otherwise this method returns the
     * number of bytes actually read.
     *
     * <p>
     * Subclasses of this class are encouraged, but not required, to attempt to read as many bytes
     * as possible in the same fashion.
     *
     * @param b destination buffer.
     * @param off offset at which to start storing bytes.
     * @param len maximum number of bytes to read.
     * @return the number of bytes read, or <code>-1</code> if the end of the stream has been
     *         reached.
     * @exception IOException if this input stream has been closed by invoking its {@link #close()}
     *            method, or an I/O error occurs.
     */
    public int read(byte b[], int off, int len) throws IOException {
        getBufIfOpen(); // Check for closed stream
        if ((off | len | (off + len) | (b.length - (off + len))) < 0) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return 0;
        }

        int n = 0;
        for (;;) {
            int nread = read1(b, off + n, len - n);
            if (nread <= 0)
                return (n == 0) ? nread : n;
            n += nread;
            if (n >= len)
                return n;
            // if not closed but no bytes available, return
            InputStream input = in;
            if (input != null && input.available() <= 0)
                return n;
        }
    }

    /**
     * See the general contract of the <code>skip</code> method of <code>InputStream</code>.
     *
     * @exception IOException if the stream does not support seek, or if this input stream has been
     *            closed by invoking its {@link #close()} method, or an I/O error occurs.
     */
    public long skip(long n) throws IOException {
        getBufIfOpen(); // Check for closed stream
        if (n <= 0) {
            return 0;
        }
        long avail = count - pos;

        if (avail <= 0) {
            // If no mark position set then don't keep in buffer
            if (markpos < 0)
                return getInIfOpen().skip(n);

            // Fill in buffer to save bytes for reset
            fill();
            avail = count - pos;
            if (avail <= 0)
                return 0;
        }

        long skipped = (avail < n) ? avail : n;
        pos += skipped;
        return skipped;
    }

    /**
     * Returns an estimate of the number of bytes that can be read (or skipped over) from this input
     * stream without blocking by the next invocation of a method for this input stream. The next
     * invocation might be the same thread or another thread. A single read or skip of this many
     * bytes will not block, but may read or skip fewer bytes.
     * <p>
     * This method returns the sum of the number of bytes remaining to be read in the buffer (
     * <code>count&nbsp;- pos</code>) and the result of calling the
     * {@link java.io.FilterInputStream#in in}.available().
     *
     * @return an estimate of the number of bytes that can be read (or skipped over) from this input
     *         stream without blocking.
     * @exception IOException if this input stream has been closed by invoking its {@link #close()}
     *            method, or an I/O error occurs.
     */
    public int available() throws IOException {
        return getInIfOpen().available() + (count - pos);
    }

    /**
     * See the general contract of the <code>mark</code> method of <code>InputStream</code>.
     *
     * @param readlimit the maximum limit of bytes that can be read before the mark position becomes
     *        invalid.
     * @see java.io.BufferedInputStream#reset()
     */
    public void mark(int readlimit) {
        marklimit = readlimit;
        markpos = pos;
    }

    /**
     * See the general contract of the <code>reset</code> method of <code>InputStream</code>.
     * <p>
     * If <code>markpos</code> is <code>-1</code> (no mark has been set or the mark has been
     * invalidated), an <code>IOException</code> is thrown. Otherwise, <code>pos</code> is set equal
     * to <code>markpos</code>.
     *
     * @exception IOException if this stream has not been marked or, if the mark has been
     *            invalidated, or the stream has been closed by invoking its {@link #close()}
     *            method, or an I/O error occurs.
     * @see java.io.BufferedInputStream#mark(int)
     */
    public void reset() throws IOException {
        getBufIfOpen(); // Cause exception if closed
        if (markpos < 0)
            throw new IOException("Resetting to invalid mark");
        pos = markpos;
    }

    /**
     * Tests if this input stream supports the <code>mark</code> and <code>reset</code> methods. The
     * <code>markSupported</code> method of <code>BufferedInputStream</code> returns
     * <code>true</code>.
     *
     * @return a <code>boolean</code> indicating if this stream type supports the <code>mark</code>
     *         and <code>reset</code> methods.
     * @see java.io.InputStream#mark(int)
     * @see java.io.InputStream#reset()
     */
    public boolean markSupported() {
        return true;
    }

    /**
     * Closes this input stream and releases any system resources associated with the stream. Once
     * the stream has been closed, further read(), available(), reset(), or skip() invocations will
     * throw an IOException. Closing a previously closed stream has no effect.
     *
     * @exception IOException if an I/O error occurs.
     */
    public void close() throws IOException {
        byte[] buffer;
        while ((buffer = buf) != null) {
            if (bufUpdater.compareAndSet(this, buffer, null)) {
                InputStream input = in;
                in = null;
                if (input != null)
                    input.close();
                return;
            }
            // Else retry in case a new buf was CASed in fill()
        }
    }
}
